Conventional turbomolecular vacuum pumps include a housing having an inlet port, an interior chamber containing a plurality of axial pumping stages, and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high speed by a motor to pump gas between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include nine to twelve axial pumping stages.
Variations of the conventional turbomolecular vacuum pump, often referred to as hybrid turbomolecular vacuum pumps, have been disclosed in the prior art. In one prior art configuration, one or more of the axial pumping stages are replaced with molecular drag stages which form a molecular drag compressor. This configuration is disclosed in U.S. Pat. No. 5,238,362, issued Aug. 24, 1993 and assigned to Varian Inc. A hybrid vacuum pump including an axial turbomolecular compressor and a molecular drag compressor in a common housing is sold by Varian, Inc. Molecular drag stages and regenerative stages for hybrid vacuum pumps are disclosed in the U.S. Pat. No. 5,358,373, issued Oct. 25, 1994 and assigned to Varian Inc. A gradual change in the design of the stators of the axial pumping stages is also disclosed in the U.S. Pat. No. 5,358,373. Other hybrid vacuum pumps are disclosed in the U.S. Pat. No. 5,074,747, issued Dec. 24, 1991, the U.S. Pat. No. 5,848,873, issued Dec. 15, 1998; and the U.S. Pat. No. 6,135,709, issued Oct. 24, 2000. The disclosed hybrid vacuum pumps use existing impeller types and switch abruptly from one impeller type to another.
Conventional molecular drag stages include a rotating disk, or impeller, and a stator. A pumping surface of the rotating disk is flat and smooth. The stator defines a tangential flow channel and an inlet and an outlet for the tangential flow channel. A stationary baffle, often called a stripper, disposed in the tangential flow channel separates the inlet and the outlet. As is known in the art, the momentum of the rotating disk is transferred to gas molecules within the tangential flow channel, thereby directing the molecules toward the outlet. Molecular drag stages were developed for molecular flow conditions.
Another type of molecular drag stage includes a cylindrical drum that rotates within a housing having a cylindrical interior wall in close proximity to the rotating drum. The outer surface of the cylindrical drum or the wall is provided with a helical groove. As the drum rotates, gas is pumped through the groove by molecular drag.
U.S. Pat. No. 6,607,351, issued Aug. 19, 2003 and assigned to Varian Inc., discloses hybrid turbomolecular vacuum pumps wherein the impellers of successive stages are configured with a surface topography for efficient operation at progressively higher pressures. The surface topography may include a roughened or a grooved pumping surface.
A regenerative vacuum pumping stage includes a regenerative impeller which operates within a stator that defines a tangential flow channel. The regenerative impeller includes a rotating disk having spaced-apart radial ribs at or near its outer periphery. Regenerative vacuum pumping stages were developed for viscous flow conditions.
All of the known prior art hybrid turbomolecular vacuum pumps have included one or more molecular drag stages wherein the impeller is a rotating disk having a flat surface or is a cylindrical drum. These stages require rotor-stator gaps of about five to eight thousandths of an inch to achieve a desired compression ratio. Maintaining such small gaps while minimizing the risk of contact between the rotor and the stator in several stages requires extremely tight tolerances and results in high manufacturing cost.
Accordingly, there is a need for improved hybrid turbomolecular vacuum pumps.